Thermos Best i Test

Thermos

Last updated May 2018

Full of hot chocolate for a day out on the ski slopes, coffee for your commute or warming soup for a hike through the forest - a small thermos flask is useful in many different circumstances. The biggest advantage of a small thermos is that you always have a hot drink with you so you don't have to look for cafés or shops. Another is that you save money by taking your own drinks from home.
There are currently a range of different thermoses, with traditional screw cups, snap lids or spouts to drink from, in classic designs in steel grey or with innovative forms and colourful patterns. There are also thermos jugs intended to be used when you have lots of guests, and small thermoses that contain only a single cupful.

We have tested relatively small, light thermoses that have capacities around 500 ml, which are easy to take with you and suitable for one or two people. All are made from stainless steel, which is a tough and thoroughly tested material for thermoses. We looked at the most popular models on the market, some of them genuine classics that have been manufactured for several decades.
Some thermoses are primarily intended to keep coffee hot during a short walk to the park with the pushchair, and if you want something like this you can pretty much buy any thermos, because most of them can easily cope with this kind of task. However, we require a little more of the models in our test. We want a thermos to keep liquid hot for a longer hike in the forest, including in the winter, to tolerate being dropped on the ground, to be suitable for use by children or someone wearing gloves, and not to leak into your rucksack. It also has to be easy to wash, ideally in a dishwasher, and not to consist of too many separate parts.

Our test shows that modern smaller stainless steel thermoses are generally very good. Satisfyingly, all of the thermoses we tested, even the ones that didn't come out on top, are of relatively high quality. None of them broke, even if a few ended up with small dents. None of them leaked, and the majority were pretty good at keeping the contents hot. And that means they're keeping their promises. Because "thermos" comes from a Greek word meaning "heat".

Products tested

How the test was made

We carry out our tests ourselves and test all products as they are intended to be used in reality. We first took the thermoses out into the wilderness, where they were used by two outdoor-loving families. The families were asked to open, pour, drink from, drop and handle the thermoses in all possible ways, with and without gloves and with both small and large hands.
We then carried out more controlled tests both indoors and out, such as filling each thermos with boiling water and measuring the temperature after 30 minutes, one, two and four hours. We also stood them upside down to see if they leaked.

In our assessment we focused on the following areas:

Heat retention capacity: How effectively does the thermos keep the contents hot?

Construction: How impact and shock resistant is the thermos? How well does it prevent leakage? How well designed is it? How heavy is it and how big is the opening you drink from?

Ease of use: How light, neat and stable is the thermos? Can you remove and replace the lid even with damp gloves or weak hands? Can thermoses with snap lids be resealed easily? Does it fit into a smaller rucksack? How easy is it to clean? Is it dishwasher safe? How clear are the instructions?

We gave each thermos a score according to its value for money; in other words, how good it is in each area in relation to its price tag. We thus have higher expectations of an expensive product than a cheaper one, and vice versa. We counted thermoses costing less than £25 as budget class, and those costing between £25 and £40 as medium class. If they cost more than this, we counted them as premium buys.

Thermos King Flask

Classic thermos that you can rely on all day long

Price class: Medium External diameter: 70 mm Diameter opening: 42 mm Height: 250 mm Weight: 360 grams Volume: 470 ml Heat retention capacity: 100 -> 82 °C after 4 hours at room temperature, 100 -> 90 °C after 1 hour at -10 °C

We've chosen the Thermos King Flask as our best in test. This is a true classic, made in a factory established in 1929 by the Thermos company after which all thermos flasks are named. The Thermos King Flask, with its attractive retro style and metallic paint, is made from uncrushable stainless steel and copes with our leak and drop tests without problems. The King Flask maintains high temperatures throughout our tests, both outdoors and indoors, while the outside remains cool. The lid is easy to use even for children and elderly people, and can be removed and replaced while wearing gloves. The opening could be larger to facilitate cleaning, however. The Thermos King Flask is ideal for someone who wants an attractive thermos that does what it promises, and does it well.

+Very good heat retention capacity
Slightly narrow opening makes cleaning difficult
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5 of 5

Stanley Adventure Vacuum Bottle

Useful, timeless thermos

Price class: Medium External diameter: 70 mm Diameter opening: 43 mm Height: 240 mm Weight: 513 grams Volume: 500 ml Heat retention capacity: 100 -> 81.1 °C after 4 hours at room temperature, 100 -> 88.7 °C after 1 hour at -10 °C

The Stanley Adventure Vacuum Bottle is a substantial thermos in classic style, manufactured in stainless steel. This thermos was invented way back in 1913, but the design doesn't seem to have aged and the thermos keeps warm well both during indoor and outdoor testing. In addition, it's leak proof, tolerates being dropped and is easy to manage even for younger children. The construction is simple with a screw top and a normal thermos mug. The only thing we found to be slightly negative is that the thermos opening is a bit too small, which makes cleaning more difficult. The Stanley Adventure Vacuum Bottle is quite simply a stylish, reliable classic for every trip out.

+Very good heat retention ability
Slightly difficult to clean
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4.5 of 5

Best price at:

Isosteel Quickstop

Faithful friend for a low price

Price class: Budget External diameter: 75 mm Diameter opening: 31 mm Height: 270 mm Weight: 440 grams Volume: 500 ml Heat retention capacity: 100 -> 81.6 °C after 4 hours at room temperature, 100 -> 89.9 °C after 1 hour at -10 °C:

Quickstop is made by the German company Isosteel, and is a low priced and relatively light but still sturdy thermos in stainless steel with a screw cap and thermos mug. This is a flask with no frills except the leak protection that works automatically when the mug is screwed on. In other words, it's just a standard thermos. But that's saying quite a lot. The Isosteel Quickstop stays cool on the outside and has top class heat retention capacity. The only disadvantages are that the opening's a bit small, which makes it hard to clean, and that the lid is rather slippery to handle when you're wearing gloves. It copes with the leak test without problems, but the thermos ends up with two small dents when it's dropped on the ground, which reduces the overall score slightly. However, the Isosteel Quickstop is overall a good value for money budget thermos for everyday use.

+Very good heat retention capacity
Slightly slippery lid, small opening makes cleaning difficult, not drop tolerant
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4 of 5

Primus Trailbreak Vacuum Mug

Substantial and stable thermos

Price class: Medium External diameter: 70 mm Diameter opening: 88 mm Height: 262 mm Weight: 631 grams Volume: 500 ml Heat retention capacity: 100 -> 76 °C after 4 hours at room temperature, 100 -> 87 °C after 1 hour at -10 °C

The Primus Trailbreak Vacuum Mug, from Swedish company Primus, is a large, stable thermos in stainless steel, with silicon gaskets that mean the cup doesn't leak, and a generous opening. It includes two lids as well as the actual drinking cup - a normal lid and a snap lid for when you want to drink without taking the lid off. This is a nice idea, but the snap lid isn't as easy to keep clean and breaks more easily than a standard cup. It also means you have a lot of parts to keep track of. Despite the fact that the outer cup lid on the Primus has grip-friendly grooves, it's quite difficult for anyone with small hands to open. The thermos has a cool outside and is robust enough to cope with both drop testing and leak testing. The Primus Trailbreak Vacuum Mug is a good buy for anyone who wants a substantial thermos with a choice of different types of cup.

+Good heat retention capacity, generous opening that facilitates cleaning
Slightly fiddly lid, too many parts
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3.5 of 5

Hydro Flask Wide Mouth Flex

Your best friend for short trips

Price class: Medium External diameter: 70 mm Diameter opening: 58 mm Height: 200 mm Weight: 343 grams Volume: 532 ml Heat retention capacity: 100 -> 68.3 °C after 4 hours at room temperature, 100 -> 81.5 °C after 1 hour at -10 °C

The Hydro Flask Wide Mouth Flex is a small, lightweight and easy to manage thermos made from stainless steel, the outside of which doesn't get hot. The large opening makes it easy to clean. The lid has a handy rubber handle that's easy to get a grip of, even for cold, fumbling hands after a day on the ski slopes. The Wide Mouth Flex has no problems with our drop or leak testing. However, the thermos goes cold relatively quickly, both indoors and in winter temperatures outdoors. The Hydro Flask Wide Mouth Flex is therefore most suitable for short adventures or for your morning coffee on the way to work.

+Practical lid with handle, generous opening facilitates cleaning
Mediocre heat retention capacity
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3 of 5

Best price at:

Klean Kanteen Insulated Classic

For trend lovers on the go

Price class: Medium External diameter: 67 mm Diameter opening: 44 mm Height: 216 mm Weight: 258 grams Volume: 355 ml Heat retention capacity: 100 -> 67.8 °C after 4 hours at room temperature, 100 -> 83.5 °C after 1 hour at -10 °C

Klean Kanteen is making a major investment in manufacturing water bottles and thermoses in stainless steel instead of plastic, with a view to reducing environmental impact. The lid on the Insulated Classic thermos is easy to use and the flask doesn't get hot on the outside or leak. However, it did get a dent during our drop testing. The thermos opening is rather small, which makes cleaning difficult, and the thermos doesn't keep drinks hot for very long. However, the Insulated Classic gets a plus for the innovative design and the striking colours. Klean Kanteen's Insulated Classic is above all a cool thermos that feels like it was developed for city-dwellers with a caffè latte in one hand and a laptop in the other.

+Fun design, ambitious environmental goals
Mediocre heat retention capacity, hard to clean, not impact resistant
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3 of 5

Eva Solo To Go Cup

Simple everyday dishwasher-safe thermos

Price class: Medium External diameter: 60 mm Diameter opening: 65 mm Height: 200 mm Weight: 275 grams Volume: 350 ml Heat retention capacity: 100 -> 54.2 °C after 4 hours at room temperature, 100 -> 75.2 °C after 1 hour at -10 °C


The Eva Solo To Go Cup is a smaller thermos in stainless steel with a silicon snap lid, which is great if you don't want to have to unscrew a cup. However, the snap lid means a bigger risk of leaks, because if there's any remaining liquid in the lid it may leak out. Despite a useful strap in the cup, it's a little difficult for smaller children to open. The thermos is dishwasher-safe, which is unusual and is a big plus point for anyone who doesn't like washing dishes by hand. It doesn't leak when laid on its side, and nor does it break when dropped on the ground. However, the outside gets a little warm when filled up with hot liquid and the thermos doesn't stay hot for very long. The Eva Solo To Go Cup isn't something you'll want to take on a full day trip, but just as the name implies it's fine as an easy-to-manage thermos to fill with coffee and shove in your bag when you're on the go.

+Practical carrying strap, dishwasher-safe
Very limited heat retention capacity, slightly fiddly lid with some risk of leakage
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2.5 of 5